Integrated deck structures are prefabricated decks designed to be installed as a unit onto a platform substructure at an offshore location. These substructures provide a framework for anchoring and supporting the deck, and are normally either fixed or floating. Fixed substructures are attached to the ocean floor, typically by means of a gravity base or a piled foundation. Floating substructures are sufficiently buoyant that their own weight, and that of an integrated deck structure placed on the substructure, is carried by the buoyancy. Floating substructures are typically maintained in position by an anchoring system.
When mating an integrated deck structure with an offshore platform substructure, the deck may be transported to the substructure by means of a barge. For most applications, a single barge may be used; however, in mounting deck structures onto single leg substructures such as those used in arctic climates, multiple barges may be required. Typically, the barge is positioned proximate the substructure and ballasted to lower the integrated deck structure onto the substructure and transfer its load thereto. Once the integrated deck is in place with the load transferred to the substructure, the barge may be disengaged and taken back to shore.
Major complications can arise during this mating procedure, due in large part to the significant relative vertical motion between the integrated deck and the substructure as the deck i transferred onto the substructure. The shock of the initial contact between the integrated deck structure and the substructure can cause appreciable damage to both structures and result in unwanted delays and expense to repair or replace any equipment so damaged. The larger the moving loads, the greater the possibility and severity of damage due to shock loads at impact.
Prior art techniques for reducing the shock loads primarily involve placing some form of spring system between the integrated deck structure and the substructure. One approach to this problem is illustrated by the system described in U.S. Pat. No. 4,413,926, issued on Nov. 8, 1983 to Ninet, in which an elastomeric spherical shock absorber is placed between the integrated deck structure and a probe which extends below the integrated deck structure. The probe is lowered into a receptacle in an upwardly extending leg of the substructure, and the integrated deck structure is then lowered onto the probe. As the load of the integrated deck structure is transferred to the probe and hence to the upwardly extending leg of the substructure, the spherical shock absorber absorbs the impact load and is squashed. A similar apparatus described in U.S. Pat. No. 4,436,454, issued Mar. 13, 1984 also to Ninet, substitutes an elastomeric block as a damping unit in place of the elastomeric sphere previously described.
An alternative approach to this problem is that illustrated by the system described in U.S. Pat. No. 4,222,683, issued on Sept. 16, 1980 to Schaloske et al. Schaloske et al., in one embodiment, describe an apparatus in which the desired shock absorbing function is performed by springs mounted between a downwardly extending leg of the integrated deck structure and a guide ring about the lower end of the leg. These springs absorb the shock of impacts between the guide ring and an upwardly extending leg of the substructure. The springs and guide ring are supported from the downwardly extending leg on extended hydraulic cylinders. The hydraulic cylinders are subsequently retracted to relieve the springs from compression after the integrated deck is in place on the substructure.
Another approach to this problem, also involving a hydraulic cylinder, is that illustrated in a paper entitled Model Testing the Offshore Installation of an Integrated Deck by R. G. Standing et al, published at the 6th International Symposium on Offshore Mechanics and Arctic Engineering (OMAE), Houston, Texas, March 1987. This particular system is described as having hydraulic cylinders which are activated to insert stab pins from the downwardly extending legs of the integrated deck into sleeves in the upwardly extending legs of the substructure, thereby eliminating relative horizontal motion between the deck and the substructure. The deck is then lowered onto the substructure and the impact loads are absorbed by a vertical elastomer in the upwardly extending leg of the substructure.
It will be appreciated that the above-discussed prior art systems rely on linearly-responsive shock absorbers or springs to absorb the impact loads. These prior art systems consequently suffer from a serious shortcoming stemming from the dual function of such springs. The springs must, first, absorb the load impact and, second, assume the full load of the integrated deck structure. A very soft linear spring would provide the ideal minimal shock loading at impact. But such a soft linear spring takes a prohibitively large amount of deflection before assuming the full load. A very stiff linear spring, on the other hand, would assume the full load with only a short deflection, but would result in prohibitively severe shock loading at impact. Ideally, a spring system for use in mating an integrated deck structure with an offshore substructure would be very soft at impact and then would smoothly transition to a very stiff response as compression proceeded and it assumed the load of the structure, i.e. the spring would be non-linear in its response. The present invention is aimed at providing a non-linear spring system having these characteristics and consequently overcoming the shortcomings of the prior art systems.